1. Field of the Invention
The present invention relates to a tracking antenna, for example, mounted on a vehicle such as an automobile for controlling to direct a beam toward a communication satellite for a satellite communication.
2. Description of the Related Art
In general, there has been known a tracking antenna apparatus of this type for tracking a communication satellite in which the bearing angle of a vehicle to be carried is detected by an azimuth detection sensor such as a magnetic compass, and a beam direction is sequentially corrected in response to detection information. However, since this method is vulnerable to various kinds of noise inputs due to its arrangement, the reliability of directional control is deteriorated. In addition, this method requires such a sensor apparatus.
On the other hand, there has been also known an antenna apparatus of the type for stepwisely tracking a communication satellite by periodically monitoring the intensity of a reception signal, comparing the intensities of the respective reception signals, and controlling to direct a beam so that the intensity of the reception signal becomes maximum. However, generally low signal-to-noise ratio in satellite communication requires a relatively long measuring time to accurately detect the intensity of the reception signal. Therefore, it is difficult to cope with an abrupt direction change of a vehicle.
There has been also known an antenna apparatus of the type having two independent antenna systems in which the phase difference of the reception signals of the two antennas is obtained and a beam direction is controlled to be directed in response to the phase difference, thereby rapidly and accurately tracking a satellite. However, since this type needs two antenna systems, and it is necessary to obtain sufficient reception gain at each antenna, the entire arrangement is inefficiently increased in scale.
Such circumstances are the same, regardless of the types such as a mechanical scanning type, an electronic scanning type, etc.